Newsflash: Mortgage calculator vs what I know I can afford?

We are in the beginning stages of looking to buy a house. We plan on talking to a mortgage broker in the next week or so. Question is when just looking around online and playing with mortgage calculators on websites trying to see what we can afford is showing a very low amount , nothing decent in our area for that price. It tells me what I can afford is x amount when I know we can afford more monthly since we are paying that currently in rent and doing fine. Any idea how accurate those online tools are? And do banks take in to consideration what you can afford based off what you are already paying in rent?